The development of digital networking combined with that of multimedia processing in the past two decades has resulted in ever-increasing digital content sharing. This raises specific issues both for commercial content providers who lose a significant source of revenue when a single piece of content is freely shared by consumers or published over the internet and for end users desiring to limit access of their personal content to their private network of friends and relatives for privacy concerns.
Media content protection against unauthorized content access, redistribution or use has long been based upon encryption of the content distribution packets. However, ultimately, the content has to be in the clear to be accessed by the end user, so content protection against unauthorized content access, redistribution or usage is a fundamentally ill-posed problem. In practice though, thanks to the reality constraints of designing a perfect hacking system at a reasonable cost and in a reasonable time, it is still possible to address this problem in the real world by designing pragmatic content protection systems based on a smart serialization of security barriers that have to be regularly adapted to the hacking threats subject to fast evolving reverse engineering technical capabilities and cost accessibility. Examples of such barriers are:                tamper-resistant hardware designs—e.g. smartcards and secure descrambling chipsets that are typically serialized into a secure STB architecture design;        tamper-resistant software designs—for instance based upon key cascading, white-box cryptography and implementation diversity that are typically combined into an obfuscated software architecture design.        
In broadcast applications, initial systems were designed to carefully protect the rights management keys, then more and more directly the content keys. However, in the end the content still ends up in the clear, so the next security engineering challenge lies in the design of content protection schemes as close as possible to the content decoding stage, either in the decoder silicon implementation of a consumer device chipset (such for instance as a Set-Top-Box STB, a television, a tablet or a handheld device) or in the software decoder implementation of a media player for mobile or PC devices.
As the trade-off between content protection design complexity and end-to-end content security efficiency may no longer be economically viable, content tracing is of particular interest. To this end, the content is individually marked to enable traceability of its leakage source and consequently enable legal or technical answers targeted at that source specifically.
Content tracing may also be used in combination with conventional content protection mechanisms. It is of particular importance in the framework of software implementations in open devices such as personal computers that intrinsically facilitate reverse engineering of the security design, algorithms and keys.
A number of solutions have been proposed to address the content tracing problem, such as watermarking the content to embed an invisible signal carrying identification information into the audio, picture or video signal. The main drawback of those tracing solutions is that in a conventional broadcast application, they either require reprocessing the content before its broadcast with a significant side channel transmission i.e. extra processing, storage and/or bandwidth at the distribution side, or alternately a complex signal processing implementation at the receiver side that has to be carefully designed again against receiver hacking.
Methods for embedding a watermark in media content are known in the state of the art. The owner of content adds the watermark to his content and distributes the content. In some methods a content owner who later finds a copy of content that he has distributed, is able to identify his watermark and simply conclude that the particular copy is a pirate copy, without being able to pinpoint where it came from. In other methods the watermark may allow for the source of an illegally distributed content to be identified. According to such methods the owner of the content issues media content, watermarked in a particular fashion, to a particular recipient. If the owner later recovers the content from a source other than the recipient, then he can extract the watermark thus identifying the recipient. He then concludes that the recipient has illegally retransmitted the content. The watermark is added to the content at the time of encoding. One of the requirements of a watermark is that it should not reduce the quality of the signal which is watermarked. Another requirement, although conflicting with the first requirement is that the watermark should be robust in that it should not be easy to remove by a third party. As is known, an encoding algorithm may include a frequency domain transform operation such as a forward discrete cosine transform. The encoding algorithm may also include a quantization process. On the decoding side, it is known for the decoding algorithm to include a de-quantization process, a frequency domain transform operation, such as an inverse discrete cosine transform, which matches or best approximates a match of the frequency domain transform operation in the encoding algorithm (the forward discrete cosine transform. The decoding algorithm may further comprise post-processing steps such as filtering and oversampling.